Solid-state transformations, catalyst

In the case of selective oxidation catalysis, the use of spectroscopy has provided critical Information about surface and solid state mechanisms. As Is well known( ), some of the most effective catalysts for selective oxidation of olefins are those based on bismuth molybdates. The Industrial significance of these catalysts stems from their unique ability to oxidize propylene and ammonia to acrylonitrile at high selectivity. Several key features of the surface mechanism of this catalytic process have recently been descrlbed(3-A). However, an understanding of the solid state transformations which occur on the catalyst surface or within the catalyst bulk under reaction conditions can only be deduced Indirectly by traditional probe molecule approaches. Direct Insights Into catalyst dynamics require the use of techniques which can probe the solid directly, preferably under reaction conditions. We have, therefore, examined several catalytlcally Important surface and solid state processes of bismuth molybdate based catalysts using multiple spectroscopic techniques Including Raman and Infrared spectroscopies, x-ray and neutron diffraction, and photoelectron spectroscopy. 

Continuous exposure of catalysts to high temperatures may cause an alteration in its components and gradually lead to its deactivation. Thermal degradation may have an undesirable impact on both the catalyst substrate and noble metal load in various ways. Thermal degradation covers two phenomena sintering and solid-state transformation. 

Temperature is the main factor that affects sintering and solid-state transformation experimental observations, however, have shown that the nature of the atmosphere in which the catalyst is heated may also play a part to an extent in the sintering process. For example, the presence of water vapor accelerates crystallization and structure modifications in oxide supports (Forzatti and Lietti, 1999). 

The discussion in the present article was restricted to catalysts active in reactions involving oxygen. We have discovered recently other solid state transformations due to spillover hydrogen. In particular, the so-called C0M0S species mentioned in hydrodesulfurization gets decomposed by spillover hydrogen produced by CopSg (37,38). 

The general conclusion of this contribution is that spillover species play a crucial role in solid-state transformations in catalysts during the catalytic reaction. The examples presented here show that a proper use of adequate additional phases, as donor of spillover species, may contribute to increase catalyst stability. 

The last step of catalyst preparation is the activation which is required for both types of materials. In this step, which often occurs in the initial stages of catalytic operation, (in situ conditioning) the catalyst is transformed into the working state which is frequently chemically and/or structurally different from the as-synthesized state. It is desirable to store free energy in the catalyst precursor which can be used to overcome the activation barriers into the active state in order to initiate the solid state transformations required for a rapid and facile activation. These barriers can be quite high for solid-solid reactions and can thus inhibit the activation of a catalyst. 

Where does all this little bit of history lead us It is certainly not just to say that some materials called "alumina" can catalyze the dehydration of ethanol to produce ethylene. It is to say that product distributions with the various dumina preparations were different, and changed in different ways as the time of operation increased. In the long run the lesson learned fi om this work was to recognize, however imperfectly that catalysts are materials, they undergo solid state transformations, they are subject to chemical attack fi om feed stream impurities, they suffer thermal stress, maybe they are susceptible to cancer - at any event, most of them eventually die. It s like a medical history why and how ... 

In the case of n-pentane oxidation, even if the preparation via the precursor N promotes higher surface area catalysts, our results show that such preparation method favours the adequate solid state transformation of the precursor into the catalyst which is optimal for the foimation of PA. 

Sintering of metals loaded on a support also leads to deactivation, for example, with Pt/alumina catalysts used in the reforming of hydrocarbons. The phenomenon has been modeled in mathematical terms by Elynn and Wanke [1974, 1975] and by Riickenstein and Pulvermacher [1973] and reviewed by Dadyburjor [1987]. The whole field of solid-state transformations was... 

Schneider, M., Winkler, S. and Bruckner, A. (2008). The Influence of Calcination Conditions on Phase Formation in Movtenbox Catalysts A Simultaneous In Situ XRD/Raman Study, SNBL Workshop on Simultaneous Raman-X-Ray Diffraction/Absorption Studies for the In Situ Investigations of Solid State Transformations and Reactions at Non Ambient Conditions, ESRF Grenoble, 18-19 June 2008. 

In this section, we discuss the high performance of the Rejo cluster/HZSM-5 catalyst, its active structure and dynamic structural transformation during the selechve catalysis, and the reaction mechanism for direct phenol synthesis from benzene and O2 on this novel catalyst [73, 107]. Detailed characterization and determination of active Re species have been conducted by XRD, Al solid-state MAS NMR, conventional XAFS and in situ time-resolved energy dispersive XAFS, which revealed the origin and prospects of high phenol selectivity on the novel Re/HZSM-5 catalyst [73]. 

The combination of these experimental findings indicate that active bismuth molybdate catalysts undergo phase transformations when exposed to reducing conditions similar to the conditions of catalysis. The phase transformations are highly dependent upon both temperature and the severity of the reducing atmosphere. However, the occurrence of solid state reactions in the catalysts suggests that the bulk structure of the catalysts plays an important role in catalytic reaction. 

---